My mother is a type 1 diabetic and she has kidney disease and was told not to take moltrin. Why?

Simply, because motrin is hard on the kidneys. Continued use can lead to kidney disease or kidney damage.

Analgesic Nephropathy (Painkillers and the Kidneys) (http://kidney.niddk.nih.gov/kudiseases/pubs/analgesicnephropathy/)

Your Mother should ask her doctor about what is safe to take.
Always ask the medical professional.